This invention relates to inserts, particularly electrically functional devices containing metal, such as transformers, sensors, solenoids, motors, relays, electrical connectors, antennas, and circuit boards, that are encapsulated with at least one layer of a thermoplastic sheet material via a compression molding process.
Inserts have been encapsulated in the past with polymeric resins for reasons of environmental protection, thermal conductivity, electrical insulation, shielding, and protection against chemicals. Such encapsulation is usually done via extrusion and injection molding processes (for example, U.S. Pat. No. 4,632,798--Eickman). The resultant encapsulated inserts are, in some cases, and especially if the inserts are electrical in nature, exposed to heat for long periods of time. It has been found that in some cases, the encapsulating layer of such encapsulated devices can crack extensively during exposure to this heat or during thermal cycling. It is believed that this cracking of the encapsulation layer occurs because of differences in the coefficient of thermal expansion, resin shrinkage, or inadequate mechanical properties, each of which could be due to fiber attrition in the resin and/or resin degradation that could occur during extrusion and/or injection molding operations.
It was an object of the present invention to develop an encapsulated insert that does not experience extensive, preferably even slight, visual cracking during thermal cycling. In the present invention, it was found that such an encapsulated insert could be prepared. More specifically, it was found that such an insert could be prepared by using at least one layer of a thermoplastic sheet material to encapsulate the insert and further by encapsulating the insert via a compression molding process.
Any insert can be encapsulated via the compression molding processes described herein using the thermoplastic sheet material described herein. Examples of inserts to which the present invention is particularly applicable include metal-containing, electrically functional devices, such as transformers and parts thereof, solenoids, sensors, motors, relays, electrical connectors, antennas, and circuit boards, and other metal devices that are exposed to heat for extended periods of time or that undergo thermal cycling.